The present invention relates to a circuit arrangement for decoding pulse spacings preferably, for filtering pulse sequences with a given spacing therebetween out of a composite signal.
In radio navigation systems, particularly in air navigation DME (Distance Measuring Equipment) systems, pulse sequences are generated which must be filtered out of a composite signal by a ground station and/or an airborne station for checking or evaluation purposes (E. Kramer, "Funksysteme fur Ortung und Navigation", Verlag Berliner Union GmbH, Stuttgart, 1973, pages 147 to 157 ["Radio Position Finding and Navigation Systems", Publisher: Berliner Union GmbH].
The ground stations transmit at predetermined time intervals an identification signal which consists of a train of pulse pairs in Morse code. After decoding, the identification pulses form a periodic pulse sequence of 1350 Hz. The code can thus be made audible for the pilot and for the personnel of the ground station.
So far, it has not been customary to monitor transmitted identification signals for completeness at the ground station. Only occasional acoustic checks of the audible Morse code have been made by personnel of the ground station. The selection of identification signals from composite signals received from the transmitter has been made by a correlator consisting of an input shift register, a reference register, and a suitable number of comparators. With a justifiable amount of circuitry, the resolution attainable with such an arrangement is limited.
To measure the slant distance between aircraft and a ground station, the airborne station transmits a pair of pulses which are acknowledged by the ground station by a pair of time-shifted reply pulses. The time interval between the two pulses is the channel identification. The latter must be recognized both by the ground station and by the airborne station. So far, a delay line has been used for this purpose which provides a delay equal to the time interval between the pulses. The accuracy achievable with such a delay line is not sufficient, however.